A meridian gyroscope suspended by a ribbon is already known from DE-A-1 941 808 and DE-A-1 941 809. Therein, the deviation of the gyroscope from the zero position of the ribbon is sensed by a pick-up device and converted into an electrical signal. This signal is applied via an amplifier to a torquer effective about the axis of the ribbon. The torquer is effective on the gyroscope to compensate for the "gyroscope restoring moment" which is effective about the axis of the ribbon on the gyroscope and which is endeavoring to orient the spin axis of the gyroscope towards the North. The exciting current for the torquer is proportional to the gyroscope restoring moment. An indicating device fed by this exciting current is provided for indicating the direction of North. In the abovementioned documents, this indicating device comprises a servo-motor by means of which the suspension ribbon is rotated until the exciting current is zero. The spin axis of the gyroscope is then oriented North whereby the gyroscope restoring moment is no longer effective. A telescope is attached to the suspension ribbon and is thus also aligned towards the North.
DE-A-2 229 431 discloses a ribbon suspended meridian gyroscope wherein the deviation of the gyroscope from the zero position of the ribbon caused by the gyroscope restoring moment is also determined by means of a pick-up and converted into an electrical signal. This signal controls via an amplifier, a servo-motor which rotates a telescope. A position indicator is coupled to the servo-motor. The signal from the position indicator is applied as a feedback signal via an amplifier to the torquer.
Finally, DE-B-21 24 357 discloses a meridian gyroscope apparatus having a gyroscope suspended by a ribbon in an intermediate housing. A pick-up produces a pick-up signal in accordance with the deviation of the gyroscope from the zero position of the ribbon. A torquer exerts on the gyroscope an opposing moment which counteracts the gyroscope restoring moment, the heavily amplified pick-up signal once again being applied to the torquer. The intermediate housing is rotatable by a servo-motor. The pick-up signal is transmitted to a storage device. After the gyroscope has been aligned, a control circuit causes a change-over switch to disconnect the pick-up signal from the pick-up and connect it to the servo-motor. A feedback circuit causes the angle through which the servo-motor is turned to be proportional to the stored pick-up signal.
The just described meridian gyroscopes permit a relatively rapid determination of the direction North. The meridian gyroscopes are mounted, as free from disturbances as possible, on a tripod which itself is firmly fixed to the ground and also protected as much as possible from vibrations. Without taking further special precautions, it is not possible to locate such meridian gyroscopes in an environment subject to disturbances such as in a vehicle.
Consequently, filtering arrangements have been developed by means of which a filtering of the signal representing a deviation from the North i.e. the torquer exciting current, takes place.
DE-A-25 45 026 discloses a ribbon suspended gyroscope having a pick-up and a torquer wherein a signal proportional to the exciting current of the torquer is digitalised in an analog-to-digital converter. The digital signal which is received at a fixed frequency is applied to a computer that forms an average value by a recursive process. The most recent recursively-determined average value is immediately supplied at the output. A filtering results from the formation of the average value. By means of the recursive formation of this average value, it is ensured that a measure for the deviation from the North is rapidly made available albeit that, in certain circumstances, it is not very precise. With increasing time however, the deviation value is constantly improved while the necessary storage capacity is kept within reasonable bounds.
A similar arrangement is disclosed in DE-A-26 18 868. There, the average value is formed from a fixed number of digital signals, that is to say the last N digital signals where N is a fixed whole number. This average value too is formed recursively.
DE-C-30 45 823 describes a method of determining the direction of North by means of a North-seeking gyroscope. A suspended body which is rotatably suspended by a universal coupling in a housing forms an outer framework. An inner framework rotatable about a vertical axis is arranged within the suspended body. A gyroscope having a horizontal spin axis is mounted within the inner framework. The inner framework is "locked" to the outer framework by a caging circuit. The caging circuit comprises a pick-up which reacts to movements of the inner framework relative to the outer framework and a torquer which is supplied with an amplified pick-up signal and exercises a torque compensating for this movement on the inner framework. In order to indicate the direction of North, this torque is measured at three different angular positions of the suspended body. The North reference angle as well as a scaling factor and the drift can be determined from the three measurements.
In the event of disturbances, e.g. vibrations in a vehicle, in which the gyroscope arrangement is located, it requires some time before the resultant measured value is available with sufficient accuracy. For three measurements, a relatively long time is thus required. On the other hand, there are some circumstances, especially military ones, where the time available for determining North is very limited.
EP-A-0 365 920 describes a filter arrangement for producing an estimated value of a measurement affected by disturbances by means of a filter which carries out a weighting of the measurement signals. This filter contains means for determining the magnitude of the disturbance and means for changing the weighting of the measurement signals in dependence on the magnitude of the disturbance. In this manner, measurement values which arise during the occurrence of a disturbance are ignored or only given little weight by the filtering action. For a caged meridian gyroscope suspended by a ribbon, its own free vibration serves as a measure of the disturbance.
DE-A-32 40 804 describes a method of determining the direction of North by means of a North-seeking gyroscope in which the gyroscope restoring moment is compensated by a counter moment of a torquer. The compensating signal is fed to a low pass filter after the formation of the average value, the differential and the amount. The output signal from the low pass filter has a constantly falling characteristic which is interrupted by disturbances. This change is detected by a check-block whose output signal then operates a switch. During undisturbed operation, the compensating signal is applied via the switch to an indicator. Upon the occurrence of a disturbance, the switch is opened.
DE-A-38 28 410 describes a filtering process for a North-seeking gyroscope having disturbance signals superimposed on the results of successive North determinations. In one time frame, it is checked whether the measurement values lie outside a predetermined maximum deviation. Only if this is not the case will the average value of the measurements taken in this time frame be supplied as corrected North indicating results.